The ash emissions are fueled by a cycle of lava dome growth and collapse. In this process, thick lava is extruded from the erupting vent near the volcano’s summit. As the dome grows the outside of the lava cools, forming a solid shell, while more lava oozes up from below. Eventually, the sides of the lava dome become too steep to be stabilized by the weak rock, and it collapses. The rock, often red-hot and riddled with gas-filled holes, disintegrates as it flows down the mountainside, creating a pyroclastic flow and ash cloud.

The video above shows a dome collapse and pyroclastic flow that occurred on Sinabung during January 2014.